What breeds are the best for meat?
The Cornish Cross is the breed used in commercial operations. They are ready to butcher in 6 to 8 weeks.
I think people generally prefer cockerels for meat birds, and that is only because they get bigger. I think pullets and cockerels are equally good for meat, as far as tenderness, when they are young.
If you want dual-purpose chickens, a breed like Rhode Island Red is good because the hens are some of the best layers, and you can butcher the roosters at around 3 months old.